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In 1983, Billy Parks wakes from a nightmare and cannot sleep. His mother Mary tells him the closet monster is imaginary. A dark apparition emerges from the closet and pulls him into darkness, spirit him away. The event hints at a supernatural threat tied to the darkness.

In 2002, Julia Lund, a psychology grad student, reunites with Billy, now grown. He tells her their night terrors are caused by something otherworldly and mentions his childhood kidnapping and two missing days. After warning her to stay out of the dark, he shoots himself. This tragedy sets Julia on a dangerous course to uncover the truth behind their fears.

Julia stays at her boyfriend Paul's apartment to grieve Billy's death. That night, she experiences unsettling signs and feels a creeping energy in the bathroom. Paul notes she is overwhelmed and suggests she might be sleep-deprived after the loss. The mounting unease foreshadows that their night terrors may be real and interconnected with dark forces.

Julia investigates a running shower and discovers a black fluid erupting from the sink drain. The bathroom mirror opens into an alternate dimension filled with mysterious creatures. Paul hears her screams and finds her alone, unable to help. The event confirms the supernatural depth of their shared night terrors.

Paul suspects she is sleepwalking and cannot explain what happened. He tries to keep things normal as Julia struggles with fragmented, conflicting memories. Her fear intensifies as events bleed into reality and friends begin to believe her warnings. The disbelief isolates Julia further and deepens the mystery.

At Billy's funeral, Julia meets two of his friends and roommates, Terry and Sam. Both had childhood night terrors and are now starting to believe Billy's claims about the danger. They share their experiences and begin to connect the pieces of the mystery. The trio agrees to investigate together.

Julia visits Billy's childhood room and discovers a drawer full of batteries. The discovery hints at patterns or experiments related to their night terrors. It deepens the mystery surrounding the creatures and their reach. It also foreshadows the importance of light in warding off the darkness.

While driving, a creature sprints across Julia's windshield, startling her and causing a near accident. The SUV's engine stalls, and she struggles to restart it. A vision of Billy appears, and she narrowly avoids a truck. The encounter confirms the threat follows her and is linked to darkness.

Terry and Sam join Julia at their apartment to study Billy's diary. Terry explains how her own night terrors began after witnessing her sister drown. She recounts dramatic episodes, including stabbing her father in the eye with a kitchen knife because she believed he was a demon. The group learns that the dark creatures are interconnected with their childhood traumas.

Julia meets a girl named Sarah, a patient of Dr. Booth, who warns that 'They' will eat you in nightmares. Sarah notices a strange mark appearing on Billy's hand, Sam's shoulder, Terry's ankle, and now Julia's forehead. The mark suggests a shared curse and a method to summon or repel the creatures. Julia studies the mark and steels herself for what's to come.

The creatures finally abduct Terry and Sam, removing two of Julia's allies. Julia's fear grows as the danger becomes personal and immediate. The loss increases the urgency to understand the mark and the power of light against the darkness. The group continues to search for means to protect themselves.

Julia discovers the mark on her forehead and removes a long black needle from her skin. She goes to Paul for help, who reacts with disbelief and drugs her with a sleeping pill. He then calls Dr. Booth, further isolating Julia and escalating her sense of danger. This marks a turning point toward institutionalization.

Julia runs to the subway and becomes trapped in the station. She boards a train home as the only passenger. The lights flicker and the train stops, and she steps off to see bulbs bursting in the tunnel as the creatures attack. She fights the beings with shards of glass, demonstrating her will to survive.

Julia is committed to Dr. Booth's mental institution, where she is attacked and pulled into the dimension she saw earlier inside a closet. No one can hear or see her scream for help as Dr. Booth and an orderly ignore her pleas. Dr. Booth closes the door, and the creatures haul Julia away, ending with her trapped in the nightmare.